David Cameron's Only Hope in 2015 Is to Destroy Nick Clegg

When Nick Clegg was making the case for fixed-term parliaments in 2010, he advanced the peculiar argument that they should be for five years rather than four because the last year or so would be taken up with electioneering, so you would only get at most four years of fully functioning government anyway. "For 12 or 18 months before an election is held, work in the House is blighted by all the parties politicking in advance of polling day," he told MPs. "Therefore, if we want Governments to govern for the long term, we think five years is the right period of time."

One might expect an incoming deputy prime minister, intent on ushering in a new politics, to do something about this dysfunctional feature of British government rather than embrace it as a politicians' birth-right. In fact, of all the tepid arguments for fixed-term parliaments, surely enabling the government to get on with the business of governing until closer to a pre-agreed election should be high on the list.

Anyway, the coalition has delivered on Mr Clegg's promise. It is hard to pinpoint precisely when the two parties began to detach themselves because they have been doing it for effect (the highly-orchestrated "differentiation strategy") almost since the beginning of their alliance. But it is quite clear now that, 14 months before the election is held, the two leaders of the government are no longer pulling in the same direction and politicking is taking over. It isn't the policy that's driven them apart; it is, for each of them, their own personal survival.

This being David Cameron and Nick Clegg, it remains gentlemanly combat - outwardly at least. But the latest briefing of the Daily Telegraph that the prime minister wants to rule out entering a post-2015 coalition hints at a much darker feud beneath the surface. For all the surface calm, they are each now trying to destroy the other. There is one big reason why Mr Cameron would want to promise he would not join another coalition in the event of a further hung parliament in 2015. It would be, as Ben Brogan points out, a rallying cry to the Tory right, re-energising those supporters who thought the party should never have entered government with the Liberal Democrats in the first place. But that is not the central argument. The main signal to voters that Mr Cameron is trying to issue is a return to the pre-2010 Lab/Con mantra that a vote for the Lib Dems is a wasted vote. This is not a new era of multi-party politics, he is saying, the choice is between Labour and the Conservatives.

For the Lib Dems, read also Ukip. It is little wonder Mr Cameron wants to advance a strategy focusing on the battle between Labour and the Tories when the biggest threat to his hopes of re-election is from another smaller party. As Lord Ashcroft's polling has shown, it is the Ukip surge that most threatens to put Ed Miliband in Downing Street. It is worth considering, in this context, how the Prime Minister may have reacted to Mr Clegg's decision to give Nigel Farage a platform for debate about the EU last week. This is the same Nigel Farage the Prime Minister desperately wants to keep out of the leaders' debates, should they be repeated, ahead of next year's general election. Yet Mr Clegg, whose election strategy is based on breaking down two-party politics, clearly sees the benefit of treating the Ukip leader as a genuine electoral prospect. After all, the Lib Dems have much less to fear from Ukip than do the other two parties - and as a consequence plenty to gain. It doesn't take much imagination to see that this is antithetical to Mr Cameron's needs.

Downing Street said the prime minister would be too busy running the country to take part in the EU debate, as clear a put-down as there can be. Lord Heseltine, the former Conservative deputy prime minister, put it rather more fruitily: "I think that it's a misjudgement frankly to equate the leadership of a party which is part of government with a protest group." It would have been difficult for Mr Cameron to say that himself, of course, because it was he who took a refuge for protest votes into government in 2010. But this is the bed he made and now must lie in: by signing up to a coalition in the first place, lending credibility to claims of a new era of power-sharing government, he has made life more difficult for himself in 2015.

If it wasn't for the fixed-term parliament, we may reasonably be expecting a general election this spring. The coalition is fragmenting, it has little left to legislate on and the prime minister needs a fresh mandate. As it is, there is still more than a year to go. Between now and the election - this 12 to 18 month period of politicking Mr Clegg was so keen to enshrine in law - the prime minister has little choice but to busy himself trying to fatally undermine his deputy. The man who kept the Tories in office for five years must now be dismissed as an irrelevance to British politics.